The present invention relates to a blade assembly with damping elements. The blade assembly includes a rotor and blades which are installed on the circumference of the rotor, extending in the radial direction and each having a blade platform, shank and a root. Arranged at least between a number of respectively adjacent blades is a damping element which, on rotation of the rotor, is frictionally connected to at least a first region of a first of the respectively adjacent blades, and to a second region of a second of the respectively adjacent blades.
Such blade assemblies are used, in particular, in turbo-engines such as gas turbines. The individual blades are composed of the blade element, the blade platform, the shank and the root, which is inserted into corresponding recesses on the circumference of the rotor. When the blade assembly is operating, undesired oscillations occur owing to various excitation mechanisms, and said oscillations can lead to premature material fatigue, and thus to a shortened service life of the blade assembly. The present invention relates to a blade assembly with damping elements which reduce these oscillations.
Damping elements which act between the individual blades have been used to reduce the oscillations of the blade assembly. These damping elements are generally loose elements which in the state of rest come to bear initially between the blade shanks on the rotor, and are pressed in the radial direction against the blade platforms of adjacent blades when the rotor is operating, owing to the centrifugal force which acts. As a result, the kinetic energy of a relative movement between the blades which is brought about by oscillations can be converted into frictional energy between the respective blade platforms and the blade element which is connected in a frictionally locking fashion. This damps the oscillations and leads to reduced oscillation loading of the blade assembly.
Such a blade assembly with damping elements is described, for example, in U.S. Pat. No. 5,156,528. In this arrangement, the edge regions of adjacent blade platforms form a recess which tapers in the radial direction and into which the damping element is pressed by the centrifugal force. The geometric shape of the damping element is matched to the shape of this recess in such a way that when the blade assembly is operating it is connected into this recess in a frictionally locking fashion. The smallest distance between the adjacent blade platforms is smaller here than the dimensions of the damping elements so that the latter cannot become detached from the blade assembly. When there is a relative movement between the adjacent blades, the movement energy is converted into frictional energy occurring at the faces which make contact with the damping element.
In addition to the shape of the damping element, which is triangular in cross section according to the illustration in this publication, other geometric shapes with which frictional engagement can be made with adjacent blade platforms are also known. However, a disadvantage of this system is that only certain higher oscillation modes of the blade assembly are converted into frictional energy with a sufficient degree of effectiveness.
A blade assembly is generally composed of 30 to 200 blades. It can be excited in a plurality of oscillation modes. For example, in the case when there are N blades, N/2+1 different natural frequencies or modes are produced in the circumferential direction of the rotor. The oscillation difference between adjacent blades is greater at higher oscillation modes. For example, in low oscillation modes only very low relative movements occur between adjacent blades, while in high oscillation modes the relative movements become very large. When oscillations are damped by converting the vibration energy into friction, it is advantageous if the relative movement between the faces which are in frictional contact with one another is as great as possible. The abovementioned technology of the damping elements which act between two adjacent blades is therefore effective only if the oscillation difference between adjacent blades is large. For this reason, the systems which are known from the prior art in this context can be used advantageously only for high oscillation modes. However, the resonances of rotating turbine blades which occur in practice are generally in the region of the lowest oscillation modes, so that the above damping elements are not sufficiently effective in this case.
In view of the above disadvantages of prior art systems, the present invention provides a blade assembly with damping elements in which the damping elements act between adjacent blades and also bring about sufficient damping even in low oscillation modes.
The blade assembly according to an embodiment of the invention has a rotor and blades which are installed on the circumference of the rotor and extend in the radial direction. Each blade is provided with a blade platform, a shank and a root. A damping element is arranged at least between a number of respectively adjacent blades, the damping element being frictionally connected, during rotation of the rotor, to at least a first region of a first of the respectively adjacent blades and a second region of a second of the respectively adjacent blades. The blade assembly includes the damping element configured and arranged between the first and second blades in such a way that the first region and the second region are located at positions which are significantly spaced apart from one another in the radial direction.
According to the invention, it has been recognized in this context that the relative movement of the faces of the damping element and of the respective blades which are in frictional contact with one another can be increased in low oscillation modes by spacing the contact faces, contact lines or contact points with the respectively adjacent blades farther apart in the radial direction. As a result of this radial distance the relative movements in low oscillation modes are increased, with the result that greater energy dissipation and thus better and more effective oscillation damping can be achieved. This technology is very advantageous in particular in the case of small relative movements between adjacent blades and in low oscillation modes, such as frequently occur. However, this technology can of course also be used for satisfactorily damping relatively large relative movements or relatively high oscillation modes.
The first and second regions are to be understood here as faces, lines or points, because the type of contact between the damping element and the blades depends on the shape of the surface of the respective contacting elements and on the operating state of the arrangement, i.e. on the rotational speed, temperature, wear and deposits. The present damping element is formed from a rigid body which is pressed against the first and second regions as a result of the centrifugal forces acting during rotation. When the damping body is pressed against regions of the adjacent blades, a portion of the energy of a vibrational movement is then converted into frictional work at the damping element.
In order to achieve an optimum damping effect, and effective dissipation of the vibration energy of low oscillation modes, the first and second regions must be spaced as far apart from one another as possible in the radial direction. The spacing in the radial direction can be preferably at least a third of the distance from the upper side of a blade platform to the surface of the circumference of the rotor. This intermediate space is taken up by the thickness of the blade platform and an upper region of the root that forms the shank. The lower region of the root is inserted in the holder or depression on the circumference of the rotor. An excessively small distance between the first and second regions leads to a situation in which the vibration energy in low oscillation modes cannot be converted into frictional energy to a sufficient degree.
Projections are provided on the blades to prevent the damping element from becoming detached while the rotor of the blade assembly is rotating. The blade platform itself can perform this function, but it is also possible to provide a separate projection on the blade in order to prevent the damping element from becoming displaced in the radial direction. Furthermore, in terms of its dimensions, the damping element should be configured in such a way that it is pressed against the adjacent blades only in the desired position when the blade assembly is operating. To this end, the damping element preferably has, in the radial plane, an elongate shape in cross section with a length which is greater than the distance between adjacent roots in the circumferential direction of the rotor. As a result of this, the damping element can be inserted between the blades in such a way that at one end it bears against the underside of the platform of the one blade, while the other end of the damping element presses against the root of the other adjacent blade at a significantly different radial position. The shape or configuration of the damping elements in the axial direction, that is to say in the direction parallel to the axis of the rotor, can be either linear or curved. This applies to all the damping elements which can be used in the arrangement according to the invention. In the present application, radial position is understood to mean the distance between a point and the axis of rotation in a radial plane. A radial plane constitutes a plane perpendicular with respect to the axis of rotation.
The precise shape of the damping elements depends on the shape, the dimensions and the distances between the individual blades of the blade assembly. The person skilled in the art will recognize that a variety of suitable shapes of the damping elements will fulfill the requirements of the invention. A number of basic shapes for suitable damping elements are presented in the exemplary embodiments given below.
The damping elements can be used particularly advantageously if their center of gravity is located near to the first or second region. The asymmetry of the damping element makes it possible to ensure that the vibration energy in the case of a relative movement between adjacent blades is converted into frictional energy in each case only at that region of contact with the damping element which is further away from the center of gravity of the damping element than the other contact region. As a result of the center of gravity of the damping element being selected to be as close as possible to one of these contact regions, there is no frictional movement, or only a very small frictional movement, at this region. This leads to an increase in the effectiveness of the conversion of energy.
In a further advantageous embodiment, the damping element has a region which is widened at one end and which, when the rotor operates, is pressed between the two platforms and thus acts as a damping element. The dimensions of this widened region and the shape of the edge regions of the platforms should be suitably matched to one another to enhance the damping action at this region. The damping element according to the invention has an extension which starts from this widened region and which extends to a region of the root which is significantly spaced apart from the platforms in the radial direction. The distribution of the center of gravity in the damping element is selected here such that the end of the extension is pressed against the root when the rotor is operating. In this embodiment, the damping properties are a result of friction between contacting surfaces that are not spaced from each other by a significant radial distance as well as contacting surfaces that are spaced from each other by a significant radial distance.
Depressions or grooves into which the damping element can be inserted or in which it engages during rotational operation and which prevent movement of the damping element in the axial direction are preferably provided on the first and/or second regions of the roots and/or blade platforms. For an optimum effect of the damping element, the first and second regions should be spaced as far apart as possible in the radial direction. Maximum spacing is achieved by the first region bearing against, or just below the platform of the first blade and just above the rotor surface on the shank of the second blade. The damping element can extend in the radial direction diagonally across the intermediate space between adjacent roots. The best damping effect can be achieved by arranging damping elements between all of the adjacent blades of the blade assembly. The mass, distribution of center of gravity, shape and material of the damping elements are selected in accordance with the desired damping properties and the properties of the rotor and the number of blades.